Intelligence officers who eavesdropped on thousands of Americans in overseas
calls under authority from President Bush have dismissed nearly all of them as
potential suspects after hearing nothing pertinent to a terrorist threat,
according to accounts from current and former government officials and
private-sector sources with knowledge of the technologies in use.

Bush has recently described the warrantless operation as "terrorist
surveillance" and summed it up by declaring that "if you're talking to a member
of al Qaeda, we want to know why." But officials conversant with the program
said a far more common question for eavesdroppers is whether, not why, a
terrorist plotter is on either end of the call. The answer, they said, is
usually no.

Fewer than 10 U.S. citizens or residents a year, according to an
authoritative account, have aroused enough suspicion during warrantless
eavesdropping to justify interception of their domestic calls, as well. That
step still requires a warrant from a federal judge, for which the government
must supply evidence of probable cause.

The Bush administration refuses to say -- in public or in closed session of
Congress -- how many Americans in the past four years have had their
conversations recorded or their e-mails read by intelligence analysts without
court authority. Two knowledgeable sources placed that number in the thousands;
one of them, more specific, said about 5,000.

The program has touched many more Americans than that. Surveillance takes
place in several stages, officials said, the earliest by machine.
Computer-controlled systems collect and sift basic information about hundreds
of thousands of faxes, e-mails and telephone calls into and out of the United
States before selecting the ones for scrutiny by human eyes and ears.

Successive stages of filtering grow more intrusive as artificial
intelligence systems rank voice and data traffic in order of likeliest interest
to human analysts. But intelligence officers, who test the computer judgments
by listening initially to brief fragments of conversation, "wash out" most of
the leads within days or weeks.

The scale of warrantless surveillance, and the high proportion of bystanders
swept in, sheds new light on Bush's circumvention of the courts. National
security lawyers, in and out of government, said the washout rate raised fresh
doubts about the program's lawfulness under the Fourth Amendment, because a
search cannot be judged "reasonable" if it is based on evidence that experience
shows to be unreliable. Other officials said the disclosures might shift the
terms of public debate, altering perceptions about the balance between privacy
lost and security gained.

Air Force Gen. Michael V. Hayden, the nation's second-ranking intelligence
officer, acknowledged in a news briefing last month that eavesdroppers "have to
go down some blind alleys to find the tips that pay off." Other officials,
nearly all of whom spoke on the condition of anonymity because they are not
permitted to discuss the program, said the prevalence of false leads is
especially pronounced when U.S. citizens or residents are surveilled. No
intelligence agency, they said, believes that "terrorist . . . operatives
inside our country," as Bush described the surveillance targets, number
anywhere near the thousands who have been subject to eavesdropping.

The Bush administration declined to address the washout rate or answer any
other question for this article about the policies and operations of its
warrantless eavesdropping.

Vice President Cheney has made the administration's strongest claim about
the program's intelligence value, telling CNN in December that eavesdropping
without warrants "has saved thousands of lives." Asked about that Thursday,
Hayden told senators he "cannot personally estimate" such a figure but that the
program supplied information "that would not otherwise have been available."
FBI Director Robert S. Mueller III said at the same hearing that the
information helped identify "individuals who were providing material support to
terrorists."

Supporters speaking unofficially said the program is designed to warn of
unexpected threats, and they argued that success cannot be measured by the
number of suspects it confirms. Even unwitting Americans, they said, can take
part in communications -- arranging a car rental, for example, without knowing
its purpose -- that supply "indications and warnings" of an attack.
Contributors to the technology said it is a triumph for artificial intelligence
if a fraction of 1 percent of the computer-flagged conversations guide human
analysts to meaningful leads.

Those arguments point to a conflict between the program's operational aims
and the legal and political limits described by the president and his advisers.
For purposes of threat detection, officials said, the analysis of a telephone
call is indifferent to whether an American is on the line. Since Sept. 11,
2001, a former CIA official said, "there is a lot of discussion" among analysts
"that we shouldn't be dividing Americans and foreigners, but terrorists and
non-terrorists." But under the Constitution, and in the Bush administration's
portrait of its warrantless eavesdropping, the distinction is fundamental.

Valuable information remains valuable even if it comes from one in a
thousand intercepts. But government officials and lawyers said the ratio of
success to failure matters greatly when eavesdropping subjects are Americans or
U.S. visitors with constitutional protection. The minimum legal definition of
probable cause, said a government official who has studied the program closely,
is that evidence used to support eavesdropping ought to turn out to be "right
for one out of every two guys at least." Those who devised the surveillance
plan, the official said, "knew they could never meet that standard -- that's
why they didn't go through" the court that supervises the Foreign Intelligence
Surveillance Act, or FISA.

Michael J. Woods, who was chief of the FBI's national security law unit
until 2002, said in an e-mail interview that even using the lesser standard of
a "reasonable basis" requires evidence "that would lead a prudent,
appropriately experienced person" to believe the American is a terrorist agent.
If a factor returned "a large number of false positives, I would have to
conclude that the factor is not a sufficiently reliable indicator and thus
would carry less (or no) weight."

Bush has said his program covers only overseas calls to or from the United
States and stated categorically that "we will not listen inside this country"
without a warrant. Hayden said the government goes to the intelligence court
when an eavesdropping subject becomes important enough to "drill down," as he
put it, "to the degree that we need all communications."

Yet a special channel set up for just that purpose four years ago has gone
largely unused, according to an authoritative account. Since early 2002, when
the presiding judge of the federal intelligence court first learned of Bush's
program, he agreed to a system in which prosecutors may apply for a domestic
warrant after warrantless eavesdropping on the same person's overseas
communications. The annual number of such applications, a source said, has been
in the single digits.

Many features of the surveillance program remain unknown, including what
becomes of the non-threatening U.S. e-mails and conversations that the NSA
intercepts. Participants, according to a national security lawyer who
represents one of them privately, are growing "uncomfortable with the mountain
of data they have now begun to accumulate." Spokesmen for the Bush
administration declined to say whether any are discarded.

New Imperatives

Recent interviews have described the program's origins after Sept. 11 in
what Hayden has called a three-way collision of "operational, technical and
legal imperatives."

Intelligence agencies had an urgent mission to find hidden plotters before
they could strike again.

About the same time, advances in technology -- involving acoustic
engineering, statistical theory and efficient use of computing power to apply
them -- offered new hope of plucking valuable messages from the vast flow of
global voice and data traffic. And rapidly changing commercial trends, which
had worked against the NSA in the 1990s as traffic shifted from satellites to
fiber-optic cable, now presented the eavesdroppers with a gift. Market forces
were steering as much as a third of global communications traffic on routes
that passed through the United States.

The Bush administration had incentive and capabilities for a new kind of
espionage, but 23 years of law and White House policy stood in the way.

FISA, passed in 1978, was ambiguous about some of the president's plans,
according to current and retired government national security lawyers. But
other features of the eavesdropping program fell outside its boundaries.

One thing the NSA wanted was access to the growing fraction of global
telecommunications that passed through junctions on U.S. territory. According
to former senator Bob Graham (D-Fla.), who chaired the Intelligence Committee
at the time, briefers told him in Cheney's office in October 2002 that Bush had
authorized the agency to tap into those junctions. That decision, Graham said
in an interview first reported in The Washington Post on Dec. 18, allowed the
NSA to intercept "conversations that . . . went through a transit facility
inside the United States."

According to surveys by TeleGeography Inc., nearly all voice and data
traffic to and from the United States now travels by fiber-optic cable. About
one-third of that volume is in transit from one foreign country to another,
traversing U.S. networks along its route. The traffic passes through cable
landing stations, where undersea communications lines meet the East and West
coasts; warehouse-size gateways where competing international carriers join
their networks; and major Internet hubs known as metropolitan area
ethernets.

Until Bush secretly changed the rules, the government could not tap into
access points on U.S. soil without a warrant to collect the "contents" of any
communication "to or from a person in the United States." But the FISA law was
silent on calls and e-mails that began and ended abroad.

Even for U.S. communications, the law was less than clear about whether the
NSA could harvest information about that communication that was not part of its
"contents."

"We debated a lot of issues involving the 'metadata,' " one government
lawyer said. Valuable for analyzing calling patterns, the metadata for
telephone calls identify their origin, destination, duration and time. E-mail
headers carry much the same information, along with the numeric address of each
network switch through which a message has passed.

Intelligence lawyers said FISA plainly requires a warrant if the government
wants real-time access to that information for any one person at a time. But
the FISA court, as some lawyers saw it, had no explicit jurisdiction over
wholesale collection of records that do not include the content of
communications. One high-ranking intelligence official who argued for a more
cautious approach said he found himself pushed aside. Awkward silences began to
intrude on meetings that discussed the evolving rules.

"I became aware at some point of things I was not being told about," the
intelligence official said.

'Subtly Softer Trigger'

Hayden has described a "subtly softer trigger" for eavesdropping, based on a
powerful "line of logic," but no Bush administration official has acknowledged
explicitly that automated filters play a role in selecting American targets.
But Sen. Arlen Specter (R-Pa.), who chairs the Judiciary Committee, referred in
a recent letter to "mechanical surveillance" that is taking place before U.S.
citizens and residents are "subject to human surveillance."

Machine selection would be simple if the typical U.S. eavesdropping subject
took part in direct calls to or from the "phone numbers of known al Qaeda"
terrorists, the only criterion Bush has mentioned.

That is unusual. The NSA more commonly looks for less-obvious clues in the
"terabytes of speech, text, and image data" that its global operations collect
each day, according to an unclassified report by the National Science
Foundation soliciting research on behalf of U.S. intelligence.

NSA Inspector General Joel F. Brenner said in 2004 that the agency's
intelligence officers have no choice but to rely on "electronic filtering,
sorting and dissemination systems of amazing sophistication but that are
imperfect."

One method in use, the NSF report said, is "link analysis." It takes an
established starting point -- such as a terrorist just captured or killed --
and looks for associated people, places, things and events. Those links can be
far more tenuous than they initially appear.

In an unclassified report for the Pentagon's since-abandoned Total
Information Awareness program, consultant Mary DeRosa showed how "degrees of
separation" among the Sept. 11 conspirators concealed the significance of clues
that linked them.

Khalid Almihdhar, one of the hijackers, was on a government watch list for
terrorists and thus a known suspect. Mohamed Atta, another hijacker, was linked
to Almihdhar by one degree of separation because he used the same contact
address when booking his flight. Wail M. Alshehri, another hijacker, was linked
by two degrees of separation because he shared a telephone number with Atta.
Satam M.A. Al Suqami, still another hijacker, shared a post office box with
Alshehri and, therefore, had three degrees of separation from the original
suspect.

'Look for Patterns'

Those links were not obvious before the identity of the hijackers became
known. A major problem for analysts is that a given suspect may have hundreds
of links to others with one degree of separation, including high school
classmates and former neighbors in a high-rise building who never knew his
name. Most people are linked to thousands or tens of thousands of people by two
degrees of separation, and hundreds of thousands or millions by three
degrees.

Published government reports say the NSA and other data miners use
mathematical techniques to form hypotheses about which of the countless
theoretical ties are likeliest to represent a real-world relationship.

A more fundamental problem, according to a high-ranking former official with
firsthand knowledge, is that "the number of identifiable terrorist entities is
decreasing." There are fewer starting points, he said, for link analysis.

"At that point, your only recourse is to look for patterns," the official
said.

Pattern analysis, also described in the NSF and DeRosa reports, does not
depend on ties to a known suspect. It begins with places terrorists go, such as
the Pakistani province of Waziristan, and things they do, such as using
disposable cell phones and changing them frequently, which U.S. officials have
publicly cited as a challenge for counterterrorism.

"These people don't want to be on the phone too long," said Russell Tice, a
former NSA analyst, offering another example.

Analysts build a model of hypothetical terrorist behavior, and computers
look for people who fit the model. Among the drawbacks of this method is that
nearly all its selection criteria are innocent on their own. There is little
precedent, lawyers said, for using such a model as probable cause to get a
court-issued warrant for electronic surveillance.

Jeff Jonas, now chief scientist at IBM Entity Analytics, invented a
data-mining technology used widely in the private sector and by the government.
He sympathizes, he said, with an analyst facing an unknown threat who gathers
enormous volumes of data "and says, 'There must be a secret in there.' "

But pattern matching, he argued, will not find it. Techniques that "look at
people's behavior to predict terrorist intent," he said, "are so far from
reaching the level of accuracy that's necessary that I see them as nothing but
civil liberty infringement engines."

'A Lot Better Than Chance'

Even with 38,000 employees, the NSA is incapable of translating,
transcribing and analyzing more than a fraction of the conversations it
intercepts. For years, including in public testimony by Hayden, the agency has
acknowledged use of automated equipment to analyze the contents and guide
analysts to the most important ones.

According to one knowledgeable source, the warrantless program also uses
those methods. That is significant to the public debate because this kind of
filtering intrudes into content, and machines "listen" to more Americans than
humans do. NSA rules since the late 1970s, when machine filtering was far less
capable, have said "acquisition" of content does not take place until a
conversation is intercepted and processed "into an intelligible form intended
for human inspection."

The agency's filters are capable of comparing spoken language to a
"dictionary" of key words, but Roger W. Cressey, a senior White House
counterterrorism official until late 2002, said terrorists and other
surveillance subjects make frequent changes in their code words. He said, "
'Wedding' was martyrdom day and the 'bride' and 'groom' were the martyrs." But
al Qaeda has stopped using those codes.

An alternative approach, in which a knowledgeable source said the NSA's work
parallels academic and commercial counterparts, relies on "decomposing an audio
signal" to find qualities useful to pattern analysis. Among the fields involved
are acoustic engineering, behavioral psychology and computational
linguistics.

A published report for the Defense Advanced Research Projects Agency said
machines can easily determine the sex, approximate age and social class of a
speaker. They are also learning to look for clues to deceptive intent in the
words and "paralinguistic" features of a conversation, such as pitch, tone,
cadence and latency.

This kind of analysis can predict with results "a hell of a lot better than
chance" the likelihood that the speakers are trying to conceal their true
meaning, according to James W. Pennebaker, who chairs the psychology department
at the University of Texas at Austin.

"Frankly, we'll probably be wrong 99 percent of the time," he said, "but 1
percent is far better than 1 in 100 million times if you were just guessing at
random. And this is where the culture has to make some decisions."